I am using Apache with PHP on a Unix server, and some days back someone changed some PHP files. As part of the investigation, I want to know where the FTP log is saved for the same server so that we can track down the responsible person.

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You are making a mistake by assuming that all file modifications are always done over FTP. It's possible that someone connected using SFTP (which does not log transfers) or edited over SSH directly.
–
grawityOct 24 '11 at 10:09

Or the server was hacked and files were uploaded through a backdoor.
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vtestOct 24 '11 at 10:11